1. Field of the Invention
This invention relates generally to radio navigation systems, and particularly to beacon equipment for such systems of the simulated Doppler type.
2. Description of the Prior Art
In British Pat. Nos. 1,225,190 and 1,234,541, known systems of the simulated Doppler type are described. In Doppler beacons according to that prior art, a source of radio frequency energy is commutated to separate radiator elements of a linear array to simulate unidirectional or bidirectional constant velocity motion of the source. Navigational information can be air-derived from these beacons, since the Doppler shift of frequency caused by commutation is proportional to the cosine of the angle which a radio receiver of the system subtends with respect to the line of apparent movement of the source.
The Doppler navigation system outlined above typically operates at a radio frequency of one or more GHz, and since the maximum Doppler frequency shift is of the order of a few KHz, it is necessary in practice to use a reference radiator at the beacon, the latter radiating a second radio frequency slightly offset from the commutated frequency, e.g., by 20 KHz. The Doppler shift to the frequency of the moving component is then detected as a change of the beat frequency between the moving component and the reference signal. Thus the indicated change of beat frequency, which bears the navigational information, is determined by the change of path difference between the two path lengths corresponding to successive commutation cycles.
With the fixed reference radiator of the above described system, this change of path length arises solely from the movement simulated by the array commutation. There is always the possibility of multipath situations occurring due to interference with the reference component being radiated from a fixed point. A multipath situation is one in which, in addition to the desired signal component (by direct propagation), there are also generated, unwanted components (by reflection). It will be apparent from knowledge of the aforementioned prior art, that the effect of multipath signal components at the airborne location is to deteriorate the accuracy of the angle determination process.